The present invention relates to recording disks. More particularly, the present invention relates to circular recording disks of exceptional flatness made of a thermoplastic resin and used for the storage of data or electronic signals in magnetic or optical form.
Recording disks such as hard disks used in the magnetic storage and retrieval of computerized information, or laser disks employing a laser light means for the retrieval of prerecorded data and information are called upon to possess ever increasing levels of performance. In view of the high level of integrity required of data files retained in the form of recording disks, there is a need for continued improvement in presently available recording disks.
Moreover, such recording disks especially laser disks must possess sufficient physical properties to resist warpage and dimensional variation when exposed to differences of humidity and temperature as well as to resist vibration or flutter due to physical stress and strain during operation. Presently available thermoplastic recording disks are easily and economically fabricated by injection molding the respective parts from thermoplastic resins, however, such recording disks may be deficient in one or more physical properties.
Suitable thermoplastic resins for use in the preparation of recording disks must possess acceptable properties of heat distortion, modulus of elasticity, and dimensional stability. A suitable resin should also possess low water absorption when exposed to elevated levels of humidity. It is believed that polymer dimensional change may result upon absorption of water vapor. Additionally it is desirable to reduce contact of water with the recording layer to prevent deterioration thereof. Resins for use in components of optical disks must further be capable of preparing moldings having suitable birefringence values so that the light beam employed in reading recorded information is not adversely affected by portions of the disk through which it passes. To satisfy this latter requirement the resin must possess suitable properties to allow for the formation of molded parts having low molded in stress. Only by successfully possessing low molded in stress are optical recording disks with suitable optical properties prepared.
Optical disks normally contain several layers in axial alignment. One such layer is a substrate layer which imparts physical integrity to the structure. Next there may be provided a recording layer such as a vacuum deposited reflective metal layer. The substrate may be pitted or etched to provide necessary surface irregularities corresponding to the recorded information while the recording layer provides a method for reading or retrieving such information. In some constructions there is additionally provided an optically transparent cover such as a solid film which is physically attached to the substrate layer substantially coextensively therewith at the circumference and axis. Generally, a covered structure has greater strength than an uncovered structure, however, additional steps in assembly of covered laser disk are required.
In either a covered or uncovered design single or double sided disks may be provided. A particularly advantageous design comprises two axially aligned halves, each half comprising a substrate layer and recording layer, arranged such that the recording layers are disposed in contact with each other in the center of the structure. The resulting structure has recording layers that are protected by outwardly facing substrate layers and does not require separately prepared covers.
In either covered or coverless designs it is critical that the thermoplastic resin employed possess extremely high dimensional stability to avoid buckling or wrinkling of the resulting structure. Particularly for larger diameter disks of extremely thin cross-sectional dimension such stability is important. For example, it is believed that thermoplastic recording disks possessing commercially suitable physical properties and having a diameter greater than 8 inch (200 mm) and a thickness less than about 0.12 inch (3.0 mm) and especially less than about 0.10 inch (2.5 mm) have been previously unattainable.
It would be desirable to provide an improved recording disk demonstrating greater reliability. In addition, it would be desirable to provide a laser or magnetic recording disk comprising a thermoplastic polymer demonstrating improved stability and reliability due to increased dimensional stability and reduced moisture absorption.